Method of manufacturing a thin film battery

ABSTRACT

In a method of manufacturing a thin film battery in a chamber, a target comprising LiCoO 2  is provided on a magnetron cathode in the chamber, and a substrate is placed facing the target. A process gas is introduced into the chamber and the process gas is energized to form a plasma to sputter the target to deposit LiCoO 2  on the substrate. An ion flux of from about 0.1 to about 5 mA/cm 2  is delivered from the plasma to the substrate to enhance the crystallinity of the deposited LiCoO 2  material on the substrate. The process gas is exhausted from the chamber. The target can also be made of other materials.

CROSS-REFERENCE

[0001] This application is a divisional of U.S. patent application Ser.No. 09/656,012, filed Sep. 7, 2000, which is incorporated herein byreference in its entirety.

BACKGROUND

[0002] The invention relates to a method of manufacturing a thin filmbattery.

[0003] A thin film battery typically comprises a substrate having one ormore thin films thereon, which may serve as, for example, currentcollectors, a cathode, an anode, and an electrolyte, that cooperate tostore electrical charge to generate a voltage. The thin film batteriestypically are less than about {fraction (1/100)}^(th) of the thicknessof conventional batteries. The thin films are typically formed by thinfilm fabrication processes, such as for example, physical or chemicalvapor deposition methods (PVD or CVD), oxidation, nitridation orelectroplating. The substrate material is selected to provide gooddielectric properties and good mechanical strength. Suitable substratematerials may include for example, oxides such as aluminium oxide andsilicon dioxide; metals such as titanium and stainless steel; andsemiconductors such as silicon.

[0004] However, conventional substrate materials often limit the abilityof the battery to store electrical energy to achieve high energy densityor specific energy levels. The energy density level is energy level perunit volume of the battery. The specific energy level is the energylevel per unit weight of the battery. Conventional batteries typicallyachieve energy density levels of 200 to 350 Whr/l and specific energylevels of 30 to 120 Whr/l. However, it is desirable to have a thin filmbattery that provides higher energy density and specific energy levelsto provide more power per unit weight or volume.

[0005] The ability to achieve higher energy levels is also enhanced byforming a crystalline cathode film on the substrate. The crystallinecathode film can also provide better charging and discharging rates.However, it is difficult to fabricate thin film batteries havingcrystalline cathode films on the substrate. Typically, the cathode is athin film deposited on the substrate in the amorphous ormicrocrystalline form, and thereafter, crystallized by annealing at hightemperatures. For example, an amorphous or microcrystalline film ofLiCoO₂ is typically annealed at about 700° C. to obtain a crystallineLiCoO₂ cathode film. However, the higher annealing temperatureconstrains the types of materials that may be used to form the otherthin films on the substrate. The other thin film materials should not,for example, soften, melt, oxidize, or inter-diffuse at annealingtemperatures. The annealing process may also generate thermal stressesthat arise from the difference in thermal expansion coefficient of thesubstrate, cathode, and current collector, resulting in delamination orpeeling off of the thin films or even the entire thin film batterystructure. Thus, conventional methods are often deficient in theirability to fabricate the crystalline cathode film of the thin filmbattery.

[0006] Thus it is desirable to have a thin film battery capable ofproviding relatively high energy density and specific energy levels. Itis also desirable to reduce the temperatures of fabrication of thecrystalline thin film materials, especially in the fabrication ofcathode comprising LiCoO₂.

SUMMARY

[0007] In a method of manufacturing a thin film battery in a chamber, atarget comprising LiCoO₂ is provided on a magnetron cathode in thechamber, and a substrate is placed facing the target. A process gas isintroduced into the chamber and the process gas is energized to form aplasma to sputter the target to deposit LiCoO₂ on the substrate. An ionflux of from about 0.1 to about 5 mA/cm² is delivered from the plasma tothe substrate to form a crystalline LiCoO₂ film on the substrate. Theprocess gas is exhausted from the chamber. The target can also be madeof other materials.

[0008] The plasma can be generated from the process gas by operating themagnetron cathode at a power density level of from about 0.1 to about 20W/cm².

[0009] The ion flux can be obtained by maintaining the substrate at apotential of from about −5 to about −200 V to cause plasma ions to beattracted to the substrate in the desired ion flux ratio. The ion fluxenhances the crystallinity of the deposited LiCoO₂ material on thesubstrate.

[0010] In one version, the substrate comprises mica, for example, a micafoil. The mica foil can be sized smaller than 100 microns to give a thinfilm battery having a high energy density.

DRAWINGS

[0011] These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription, appended claims, and accompanying drawings, whichillustrate embodiments of the present invention that may be usedseparately or in combination with one another, where:

[0012]FIG. 1 is a schematic cross-sectional view of an embodiment of athin film battery according to the present invention;

[0013]FIG. 2 is a flow chart of the method of fabricating a thin filmbattery according to another embodiment of the present invention;

[0014]FIG. 3 is a schematic diagram of the structure of a magnetronsputtering cathode apparatus according to the present invention;

[0015]FIG. 4 is an x-ray diffraction pattern of an as-deposited LiCoO₂film showing that the film is highly crystalline and with a (110)preferred orientation; and

[0016]FIG. 5 is a discharge curve of a thin film battery according tothe present invention having a crystalline LiCoO₂ cathode.

DESCRIPTION

[0017] One embodiment of a battery 10 having features of the presentinvention is illustrated in FIG. 1. The battery 10 is formed on asubstrate 12 which can be an insulator, a semiconductor, or a conductor.The substrate 12 should also have sufficient mechanical strength tosupport the thin films during processing or operational temperatures.For example, the substrate 12 can comprise silicon dioxide, aluminumoxide, titanium, or a polymer.

[0018] In one embodiment of the present invention, which may be used byitself, or in combination with any of the other features or methodsdescribed herein, the substrate 12 comprises a thickness of less thanabout 100 microns, and more preferably less than 25 microns. The thinnersubstrate 12 reduces the total weight and volume of the battery and yetis sufficiently strong to provide the desired mechanical support for thebattery structure. A preferred substrate material comprises mica, whichmay be fabricated into a thin substrate of less than 100 microns withgood tensile strength. Mica is typically a muscovite material, which isa layered silicate with a typical stoichiometry of KAl₃Si₃O₁₀(OH)₂. Micatypically has a flat six-sided monoclinical crystalline structure withgood cleavage properties in the direction of the large planar surfaces.Because of this crystal structure, mica may be split into thin foilsalong its cleavage direction to provide thin substrates having surfaceswhich are smoother than most chemically or mechanically polishedsurfaces, which is advantageous for the fabrication of thin films on thesubstrate. Chemically, mica is stable and inert to the action of mostacids, water, alkalies and common solvents. Electrically, mica has gooddielectric strength, a uniform dielectric constant, and low electricalpower loss factors. Mica is also stable at high temperatures of up to600° C. By using mica, thin substrates may be fabricated to providelighter and smaller batteries with relatively higher energy densitylevels. Mica also provides good physical and chemical characteristicsfor processing of the thin films formed on the substrate, in a CVD orPVD chamber, such as for example, a magnetron sputtering chamber.

[0019] Referring to FIG. 1, a typical battery 10 includes a firstadhesion layer 14 deposited on a substrate 12 to improve adhesion of theother thin films formed on the substrate 12. The adhesion layer 14 cancomprise a metal such as, for example, titanium, cobalt, aluminum, othermetals, or a ceramic material such as, for example, LiCoO_(x), which maycomprise a stoichiometry of LiCoO₂. A first current collector 16 isformed over the adhesion layer 14. The current collector 16 is typicallya conductive layer which may comprise a non-reactive metal such assilver, gold, platinum or aluminum. The first current collector 16 mayalso comprise the same metal as the adhesion layer 14 in a thicknessthat is sufficiently high to provide the desired electricalconductivity.

[0020] A first electrode 18 comprising an electrochemically activematerial may be deposited over the first current collector 16. Forexample, the first electrode film 18 may comprise an amorphous vanadiumpentoxide, V₂O₅, or one of several lithium intercalation compounds thatmay be deposited in thin-film form, such as crystalline TiS₂, LiMn₂O₂ orLiCoO₂. In one exemplary embodiment, a crystalline LiCoO₂ film isdeposited upon the current collector 16 by RF or DC magnetron sputteringto serve as the first electrode or cathode. An electrolyte film 20 isformed over the first electrode 18. The electrolyte film 20 may be, forexample, an amorphous lithium phosphorus oxynitride film otherwise knownas a Lipon™ film, Dupont de Nemours, Wilmington, Del. An anode or secondelectrode 22 is deposited over the electrolyte film 20 and a secondcurrent collector 24 is deposited on the second electrode 22 and thesubstrate 12. Further layers may be formed to provide additionalprotection.

[0021] In yet another embodiment of the present invention, which alsomay be used by itself, or in combination with any of the other featuresor methods described herein, the first electrode film 18 comprises acrystalline lithium metal oxide film, such as a LiCoO₂ film. Thecrystalline LiCoO₂ film can be fabricated at low temperatures preferablybelow 600° C. by a PVD process, such as RF or DC magnetron sputteringwith a high plasma density, as provided herein.

[0022]FIG. 2 illustrates the method of making a thin film batteryaccording to the present invention. In the initial step, step 100, thesubstrate is heated to about 400° C. in air for about 10 minutes toclean the substrate 12 by burning off organic materials which may beformed on the substrate 12. Subsequently, the thin film layers of thebattery are deposited on the substrate 12. One or more of the thin filmsmay be adapted to generate or store an electrical charge.

[0023] In one method, the substrate is placed in a magnetron PVD chamber150 as shown in FIG. 3, which is pumped down to 1×10⁻⁵ Torr, step 200. Asuitable substrate comprises an array of 35 mm×62 mm sheets of mica. Thechamber 150 comprises walls 155, a gas supply 158 connected to a gasdistributor 160, a gas exhaust 165, and a power supply 170 to apply apower to a target 175. A substrate fixture 180 with the substrate 12thereon is carried into the processing chamber 150 by a conveyor andpositioned facing the target 175. The substrate holding fixture 180 iselectrically isolated from the chamber walls 155 which are typicallyelectrically grounded. The process chamber 150 is separated from aloading chamber (not shown) by a slit valve (also not shown). Theprocess chamber 150 typically comprises a volume of about 24 sq ft withdimensions of about 4′×6′×1′. The sputtering targets 175 are sized about5″×25″. The process gas distributor 160 is provided for distributingprocess gas into the chamber 150. A process gas, such as for example,argon and oxygen, may be introduced into the chamber 150 to serve as thesputtering gas. The sputtering gas is maintained in the chamber 150 at apressure of from about 5 to about 25 mTorr, in step 300, and provided ata flow rate ratio of Ar/O₂ of from about 1 to about 45.

[0024] A high density plasma is generated in the chamber 150 by amagnetron sputtering cathode 185. The plasma is formed over an area thatis sufficiently large to coat the entire substrate 12, for example, anarea of about 8″× about 25″. In one version, the magnetron cathode 185comprises central magnets 110 that provide a weaker magnetic field thanthe surrounding peripheral magnets 120. Both the peripheral and centralmagnets, 110, 120 have a polarity of south facing the chamber 150 andnorth facing away from the chamber 150. In this configuration, themagnetic field 130 generated by the magnets 120 is not confined to nearthe magnetron cathode surface 185. Instead, the magnetic field lines 130extend to near the substrate 12. Secondary electrons follow the magneticfield lines to near the substrate surface to create high-density plasmain this area. In one version, the magnets 120 are arranged about aperimeter of the target 175. Thus, the distribution of plasma ions aboutthe substrate 12 may be controlled with the magnetic field 130.

[0025] To deposit a film of LiCoO_(x) on the substrate 12, a target 175comprising LiCoO₂ is installed in the chamber 150 and themagnetron-sputtering cathode 185 is operated at a power density level offrom about 0.1 to about 20 W/cm², step 400. In conjunction withoperating the cathode 185, an ion flux of from about 0.1 to about 5mA/cm² is delivered to the substrate 12 upon which the LiCoO_(x) film isbeing deposited, step 500. During deposition, a negative potential of 5to 100 V on the substrate 12 is established with respect to the plasma,step 600. The potential can be established either by using an externalpower supply or by electrically floating the substrate holding fixture180. The parameters of the deposition process are maintained until thedesired film thickness is reached, step 700. The temperature of thesubstrate 12 during the deposition process is estimated to be from about100 to about 200° C.

[0026] In one version the as-deposited LiCoO_(x) film fabricatedaccording to the present method comprises LiCoO₂ which is crystallinewith a strong (101) preferred orientation and with a small amount of(012) oriented grains. FIG. 4 shows a typical x-ray two thetadiffraction pattern of the as-deposited LiCoO₂ film showing that thefilm is highly crystalline and with a (101) preferred orientation. Thesubstrate 12 was slightly tilted when taking x-ray diffraction in orderto suppress the diffraction peaks from the mica substrate to betterreveal the property of the LiCoO₂ film. It is believed that thecrystalline material was deposited due to a combination of plasmaheating, oxygen activation and plasma enhanced nucleation and growthprocesses. The as deposited crystalline material was a good cathodematerial.

[0027] Optionally, the cathode film formed on the substrate may beannealed to further improve the quality of the cathode film. Theannealing step was found to increase the battery capacity by 10 to 20%,increase the charge and discharge current by more than 50%, and improvethe resistance to moisture. These attributes arise from the eliminationof point defects and the reduction of electrical contact resistances inthe cathode material.

[0028] Under lower gas pressure levels of about 5 mTorr, the preferredorientation changes to (012) and (104). The (012) and (104) orientedmaterial can still be used as cathode, however, with smaller energycapacity compared to the (101) oriented material. The annealing processis typically performed at a low temperature of from about 150 to about600° C.

[0029]FIG. 5 is a typical discharge curve of a 15 cm² thin film batteryof the present invention. The battery comprised a 10 m-thick micasubstrate with a crystalline LiCoO₂ cathode layer that is close to 2 μm.The capacity of the battery, as shown in FIG. 5, is about 1.9 mAh. Thus,the capacity of the cathode is calculated to be 0.07 mAh/cm²/μm, whichis close to the theoretical number for crystalline LiCoO₂. The cut offvoltage of this battery is well defined and at 3.7 V. The energy densityand specific energy of this thin film battery, including both the celland the substrate, is about 340 wh/l and 105 wh/kg, respectively. It isexpected that an energy density of more than 700 wh/l and a specificenergy of more than 250 wh/kg can be achieved by fabricating the batterycell on both front and back side of a mica substrate. The dischargecurrent of the battery was about 2 mA.

[0030] It will be understood that numerous modifications andsubstitutions can be made to the described exemplary embodiments of thepresent invention without departing from the scope of the invention. Forexample, thin film electronic devices other than batteries may befabricated using the described substrate. Also, the battery can includeother combinations of metal, nonmetal and dielectric layers and in adifferent order than that described herein. Accordingly, the exemplaryembodiments are intended for the purpose of illustration and not as alimitation.

What is claimed is:
 1. A method of manufacturing a thin film battery ina chamber comprising a magnetron cathode, the method comprising: (a)providing a target on the magnetron cathode in the chamber; (b) placinga substrate comprising mica facing the target in a chamber; (c)introducing a process gas into the chamber; (d) energizing the processgas to form a plasma to sputter the target to deposit target material onthe substrate; (e) delivering an ion flux from the plasma to thesubstrate of from about 0.1 to about 5 mA/cm² to enhance crystallizationof the target material deposited the substrate; and (f) exhausting theprocess gas from the chamber.
 2. A method according to claim 1 wherein(a) comprises providing a target comprising LiCoO₂ in the chamber.
 3. Amethod according to claim 1 comprising applying a current at a powerdensity level of from about 0.1 to about 20 W/cm² to the target whilemaintaining the substrate at a potential of from about −5 to about −200V.
 4. A method according to claim 1 wherein the substrate comprises micahaving a thickness of less than about 100 microns.
 5. A method accordingto claim 4 further comprising forming films on the mica substrate, thefilms comprising a first and second current collectors, a cathode, andan electrolyte, such that the deposited material serves as the anode. 6.A method of manufacturing a thin film battery in a chamber comprising amagnetron cathode, the method comprising: (a) providing a targetcomprising lithium metal oxide on the magnetron cathode in the chamber;(b) placing a substrate facing the target in a chamber; (c) introducinga process gas into the chamber; (d) energizing the process gas to form aplasma to sputter the target to deposit lithium metal oxide on thesubstrate; (e) delivering an ion flux from the plasma to the substrateof from about 0.1 to about 5 mA/cm² to form a crystalline lithium metaloxide film on the substrate; and (f) exhausting the process gas from thechamber.
 7. A method according to claim 6 wherein the target comprisesLiCo_(x)O_(y).
 8. A method according to claim 6 wherein the targetcomprises LiCoO₂.
 9. A method according to claim 6 wherein the targetcomprises LiMn₂O₂.
 10. A method according to claim 6 comprisingoperating the magnetron cathode at a power density level of from about0.1 to about 20 W/cm² and maintaining the substrate at a potential offrom about −5 to about −200 V.
 11. A method according to claim 6 furthercomprising applying a non-uniform magnetic field about the target in thechamber comprising a weaker central magnetic field strength and asurrounding stronger peripheral magnetic field strength.
 12. A methodaccording to claim 6 comprising introducing a process gas comprisingargon and oxygen and maintaining the process gas in the chamber at apressure of from about 5 to about 25 mTorr.
 13. A method according toclaim 6 comprising annealing the deposited material by heating thesubstrate to a temperature from about 150 to about 600° C.
 14. A methodaccording to claim 6 comprising the initial step of forming a substratecomprising mica, and forming one or more films on the substrate togenerate or store an electrical charge.
 15. A method according to claim14 wherein the mica substrate comprises a thickness of less than about100 microns.
 16. A method according to claim 14 further comprisingforming films on the mica substrate, the films comprising a first andsecond current collectors, an anode, and an electrolyte, such that thedeposited crystalline lithium metal oxide film serves as the cathode.17. A method according to claim 6 further comprising cleaning thesubstrate before depositing material on the substrate.
 18. A methodaccording to claim 17 comprising cleaning the substrate by heating thesubstrate to about 400° C. in air.
 19. A method of manufacturing a thinfilm battery in a chamber comprising a magnetron cathode, the methodcomprising: (a) providing a target comprising LiCo_(x)O_(y) on themagnetron cathode in the chamber; (b) placing a substrate facing thetarget in a chamber; (c) introducing a process gas into the chamber; (d)energizing the process gas to form a plasma to sputter the target todeposit LiCo_(x)O_(y) on the substrate; (e) delivering an ion flux fromthe plasma to the substrate of from about 0.1 to about 5 mA/cm² to forma crystalline LiCo_(x)O_(y) film on the substrate; and (f) exhaustingthe process gas from the chamber.
 20. A method according to claim 19wherein the target comprises LiCoO₂.
 21. A method according to claim 19comprising operating the magnetron cathode at a power density level offrom about 0.1 to about 20 W/cm² and maintaining the substrate at apotential of from about −5 to about −200 V.
 22. A method according toclaim 19 further comprising applying a nonuniform magnetic field aboutthe target in the chamber comprising a weaker central magnetic fieldstrength and a surrounding stronger peripheral magnetic field strength.23. A method according to claim 19 comprising introducing a process gascomprising argon and oxygen and maintaining the process gas in thechamber at a pressure of from about 5 to about 25 mTorr.
 24. A methodaccording to claim 19 comprising annealing the deposited material byheating the substrate to a temperature from about 150 to about 600° C.25. A method according to claim 19 comprising the initial step offorming a substrate comprising mica, and forming one or more films onthe substrate to generate or store an electrical charge.
 26. A methodaccording to claim 25 wherein the mica substrate comprises a thicknessof less than about 100 microns.
 27. A method according to claim 25further comprising forming films on the mica substrate, the filmscomprising a first and second current collectors, an anode, and anelectrolyte, such that the deposited crystalline LiCoO₂ film serves asthe cathode.
 28. A method according to claim 19 further comprisingcleaning the substrate before depositing material on the substrate. 29.A method according to claim 28 comprising cleaning the substrate byheating the substrate to about 400° C. in air.